Lead-bearing steel and method of manufacture

ABSTRACT

THIS INVENTION RELATES TO A TOOL-STEEL COMPOSITION CHARACTERIZED BY IMPROVED MACHINABILITY BY THE ADDITION OF .5 TO 2% LEAD IN COMBINATION WITH SULFUR IN AN AMOUNT UP TO ABOUT .3%. ARTICLES CHARACTERIZED BY IMPROVED MACHINABILITY ARE PRODUCED IN ACCORDANCE WITH THE PRESENT INVENTION BY PROVIDING A CHARGE OF POWDERED METAL OF A DESIRED TOOL-STEEL COMPOSITION, COATING THE PARTICLES WITH LEAD OXIDE, THEREAFTER HEATING SAID CHARGE UNDER CONDITIONS OF TEMPERATURE AND PRESSURE TO DECOMPOSE SAID LEAD OXIDE BY GASSIFICATION AND THEN REMOVING THE PRODUCTS OF GASSIFICATION FROM THE POWDERED METAL CHARGE TO LEAVE THE PARTICLES COATED WITH LEAD. THE LEAD-CONTAINING CHARGE OF PARTICLES IS THEN COMPACTED TO FORM A DENSE COMPACT.

United States Patent M 3,729,293 LEAD-BEARING STEEL AND METHOD OF MANUFACTURE Gary Steven, Pittsburgh, Pa., assiguor to Crucible Inc., Pittsburgh, Pa.

No Drawing. Original application Aug. 20, 1968, Ser. 753,900. Divided and this application Apr. 30, 1971, Ser. No. 131,812

Int. Cl. B22f 3/20 US. Cl. 29-1825 4 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a tool-steel composition characterized by improved machinability by the addition of .5 to 2% lead in combination with sulfur in an amount up to about .3%. Articles characterized by improved machinability are produced in accordance with the present invention by providing a charge of powdered metal of a desired tool-steel composition, coating the particles with lead oxide, thereafter heating said charge under conditions of temperature and pressure to decompose said lead oxide by gassification and then removing the products of gassification from the powdered metal charge to leave the particles coated with lead. The lead-containing charge of particles is then compacted to form a dense compact.

This application is a division of Ser. No. 753,900, Aug. 20, 1968, now abandoned.

It is typical to improve the machinability of tool steels, and particularly high speed steels, by the use of a sulfur addition within the range of, for example, .10 to .15%. This sulfur combines with the manganese present in the tool steels to form manganese sulfides, which sulfides facilitate the machining of the resulting tool steel articles. The use of sulfur for this purpose, however, permits improvements with respect to machinability of only about 25 or 30% over similar tool-steel compositions wherein sulfur is not used. In addition, the use of sulfur adds to the production costs of tool-steel articles in that during conversion of sulfurized steel from ingot to bar product additional processing over and above that required for nonsulfurized tool steel is required. Also, the manganese sulfides incident to sulfurized tool steels cause the fine cutting edges of tool steel articles, such as drills, taps, reamers, knives, cutters and the like, to be more prone to chipping at the cutting edges. This decreases tool life over similar nonsulfurized tool steels.

The use of sulfur for the purpose of improving machinability, and the attendant disadvantages thereof, characterize all the typical tool steels, which include cold-worked, hot-worked and high-speed tool steels. Cold-worked tool steels may be of the plain carbon or low-alloy variety. The latter are of higher hardenability than the plain-carbon tool steels so that they may be hardened in heavier sections, or with less drastic quenches and consequently less distortion. The hot-worked tool steels are of intermediate alloy content. They typically contain tungsten, molybdenum or vanadium for the purpose of combining with the carbon in the steel to form hard and wear-resisting carbides. High-speed tool steels, like the hot-worked tool steel compositions, contain carbide-forming elements but in larger amounts so that they are, like the hot-worked tool steels, provided with wear-resisting carbides and in addition have sufficient carbide-forming elements to promote secondary hardening, whereby the resistance to softening at elevated temperature of tools made from this steel is drastically increased. The uses of tool steels of the above-mentioned types are extremely diverse. Obviously,

3,729,293 Patented Apr. 24, 1973 ice however, the high-speed steels are used in applications requiring long-life and relatively high operating temperatures, such as for heavy cuts or high speed machining. The hot-worked tool steels are used for hot forming operations in which extreme wear-resistance and resistance to heat softening and thermal shock is necessary. Cold- Worked tool steels may be regarded as general-purpose tool steels and the compositional choice thereof is based primarily on section size, permissible distortion, intricacy of design, and the hardness and toughness requirements of the particular application. Within this class, however, higher-carbon compositions are used for applications requiring high resistance to wear and abrasion, and the lower-carbon compositions are used in applications in which resistance to shock or impact is of particular importance.

For all of the compositions as discussed above and in all of the applications for which they are used, machining is an important step in the production of final products. As such, the machining operations add considerably to the overall final-product production cost. Consequently, in the tool steels of the various types discussed above it is known to add sulfur for the purpose of improving machinability. When sulfur is added for this purpose, all of the above-mentioned tool steels are subjected to the disadvantages as discussed above caused by sulfur addition.

It is accordingly the primary object of the present invention to provide a free-machining tool-steel composition that does not rely solely on the use of sulfur for improving machinability.

A more particular object of the invention is to provide a tool-steel composition of improved machinability produced by the addition of a substantial quantity of lead that remlains in a fine uniform distribution throughout the tool stee A related object of the invention is to provide a method for producing tool-steel articles characterized by improved machinability.

A more particular related object of the invention is to provide a method for producing leaded, free-machining tool steel articles by the use of a pewdered metallurgy technique.

These and other objects of the invention, as Well as a complete understanding thereof may be obtained from the following description and specific examples.

In accordance with the present invention, the machinability of tool steels is improved by adding lead in a fine, uniform distribution throughout the steel in an amount of about .5 to 2%. Lead may be used alone within this range for the purpose of improving machinability or, alternately, it may be used in combination with a quantity of surfur up to about 3% max. It has been found that with lead contents greater than 2%, there is no further significant improvement obtained with respect to machinability. Lead contents of at least 5% are required toachieve the desired significant improvement with regard to machinability.

The composition range for a cold-Worked tool steel suitable in the practice of the present invention is as follows:

Element:

Balance.

A hot-worked tool steel composition suitable for use in the practice of the invention is as follows:

Element: Percent Carbon .3 to .5. Manganese Up to .8. Sulfur Up to .25. Chromium 2 to 7. Vanadium Up to 2. Tungsten Up to 12. Cobalt Up to 4. Molybdenum Up to 3. Iron Balance.

The following high-speed tool steel composition is suitable for the use in the practice of the invention:

Element: Percent Carbon .8 to 1.3. Silicon Up to .5. Sulfur Up to .3. Chromium 4 to 5. Molybdenum 3.5 to 9.5. Tungsten .8 to 7. Vanadium 1.0 to 5. Cobalt Up to 12. Iron Balance.

As is well known, in the above compositions containing combinations of molybdenum and tungsten, it is to be understood that these elements are interchangeable with tungsten replacing molybdenum in a ratio of 2 to 1.

In producing tool steel articles of the above composition, wherein the lead is in a fine and uniform distribution throughout the composition and any resulting article, the following method, which is in accordance with the present invention, is employed. A charge of powdered metal for example comprising particles of the desired tool-steel composition, is produced. The particles are coated with lead oxide, preferable by tumbling the particles with a quantity of lead oxide in colloidal form until, as a result of the tumbling action, all of the particles are properly coated. Thereafter, the charge is heated under conditions of temperature and pressure sufficient to decompose the lead oxide by gassification thereof. To aid in this reaction, it is preferred to provide within the particle charge a quantity of carbon, which during heating function reacts with lead oxide by the formation of carbon monoxide. During heating, the carbon monoxide and other gaseous reaction products are removed from the particle charge. The carbon may be introduced to the charge by coating the particles with lampblack. It is understood, of course, that other carbon containing materials suitable for the purpose may also be used.

During the heating step and the removal of gaseous reaction products, the particle charge is contained within a container, which may be constructed of mild steel. The container is provided with a stem that is connected to a vacuum pump to effect the removal of the gaseous reaction products. After pumping for this purpose has been completed, the container with the particle charge therein is disconnected from the vacuum pump and the connecting stem is sealed. Thereafter, in the conventional manner, the particle charge within the container is subjected to substantial pressure while at elevated temperature. In this manner, the tool-steel particle charge is compacted to a high density, typically on the order of at least about 95%. After compacting, the container is removed by pickling, machining or a combination of both. The resulting tool steel article may then be subjected to the machining and grinding operations required in the production of the desired, final tool-steel product.

By the use of the method of the invention as discussed above, it is possible to disperse lead within an amount of about .5 to 2% in a fine and uniform distribution throughout the tool-steel article.

As a specific example of the practice of the invention,

A151 M2 and M28 tool steel powders of mesh were tumbled with a quantity of colloidal lead oxide until each particle was coated with the lead oxide. During the tumbling operation, a quantity of lampblack was introduced to the charge and accordingly coated onto the particles. After tumbling, the powdered metal charges were placed in mild steel cylindrical containers having a stem connecting the container interiors to a vacuum pump. The containers were placed in a furnace and heated to a temperature of about 2150 F. Upon reaching this temperature, which took approximately one hour, the container interiors were pumped to remove the gaseous reaction products, which are substantially all carbon monoxide. The containers, which are 3 /2" diameter x 3" high, were then upset-forged and elongated repeatedly to produce extensive and thorough working throughout the particle charge. This resulted in compacts of intermediate density. These compacts were then forged in a longitudinal direction to 1%" squares, without removal of the container. In addition to the two compacts described above, a lead-free control compact with AISI M2 powder and an additional compact with AISI M2S were produced. The two compacts produced in accordance with the practice of the invention are described in the following Table I:

TABLE I AISI M2 AISI M2S Carbon content- .80% 1.06% Additions 1.2% Pb, 2% C 2% Pb Carbon content, as forged. .93% .07% Lead content, as forged 1.12% 1.71%

The compacts were tested for constant-load drill machinability, which results are reported on Table II:

TABLE II Machinability index 1 (percent) Material Test No. 1 Test No. 2 Test No. 3

Thrust at the tip of the M-inch diameter drill: 150 lbs. Drilling speed: 480 r.p.m. Depth of drilled hole: .250 inch PIM M2 (0.9% 0) standard... 100 100 100 P/M M2 (0.93% C) 1.1% Pb--- 163 166 Commercial M25 (1.0% O) 125 123 126 Thrust at the tip of the %-inch diameter drill: 225 lbs. Drilling speed: 480 r.p.m. Depth of drilled hole: 250 inch P/M M2 (0.9% 0) standard. 100 100 100 P/M M2 (0.93% C) 1.1% Pb--- 170 164 164 Commercial M28 (1.0% C) 122 115 113 Thrust at the tip of the %-inch diameter drill: lbs. Drilling speed: 590 r.p.rn. Depth of drilled hole: .250 inch It may be seen from this table that the machinability of the leaded steel produced in accordance with the present invention is far superior to that of the lead-free compact produced in a similar manner and the commercial M2S steel.

Although various embodiments of the invention have been described herein, it is obvious that other variations may be made by those skilled in the art without departing from the scope and spirit of the appended claims.

I claim:

1. A compacted powder tool steel substantially fully dense article containing lead in an amount of about .5 to 2% in a substantially fine and uniform distribution and sulfur up to about 3%.

2. -In a compacted powder hot-worked tool steel substantially fully dense article consisting essentially of, in weight percent, .3 to .5 carbon, up to .8 manganese, up to .25 sulfur, 2 to 7 chromium, up to 2 vanadium, up to 12 tungsten, up to 4 cobalt, up to 3 molybdenum, and the balance iron, the improvement comprising an addition of lead within the range of .5 to 2% 3. In a compacted powder cold-worked tool steel substantially fully dense article consisting essentially of. in weight percent, 0.9 to 1.6 carbon, up to 1.5 manganese, up to .25 sulfur, up to 12 chromium, up to 1.5 molybdenum, up to 1.5 vanadium and the balance iron, the improvement comprising an addition of lead within .th range of .5 to 2%. a

4. In a compacted powder high-speed tool steel substantially fully dense article consisting essentially of,;in weight percent, .8 to 1.3 carbon, up to .5 silicon, up to 2.3 sulfur, 4 to 5 chromium, 3.5 to 9.5 molybdenum,.8 to 7 tungsten, 1.0 to 5 vanadium, up to 12 cobalt and the balance iron, the improvement comprising an addition of lead within the range of .5 to 2%. 1

References Cited UNITED STATES PATENTS 2,914,400 11/1959 Roberts 75123 F 3,341,325 9/1969 Cloran 75212X 6 3,424,576 l/ 1969 Fogleman et a1 75123 F 3,066,391 12/1962 VOi Dahl 75212 X 2,327,805 8/1943 Koehring 75212 X FOREIGN PATENTS 538,104 7/1941 Great Britain 75123 F OTHER REFERENCES Roberts et al.: Tool Steels, Metals Park, Ohio, Amer. Soc. for Metals (1962), p. 461-5.

Clark et al.: Physical Metallurgy for Engineers, Princeton, Van Nostrand (1962), p. 283.

CARL D. QUARFORTI-LPrimary Examiner R. E. SCHAF-ER, Assistant Examiner US. Cl. X.R. 75123 F, 126 R 

